As a variety of 3C products becomes ubiquitous, the demands on touch panel also increase. In general, the touch panel can be categorized as resistive, capacitive, ultrasonic or infrared. The least expensive touch panel in the market is the resistive system, which also has the highest market share. The resistive system uses a standard glass panel, including an upper ITO conductive layer and a lower ITO conductive layer. A spacer is disposed between the two layers and the current can flow between the two layers. When in use, the pressure makes the two ITO conductive layers contact, and the electrical field change is regarded as a touch event. Finally, a signal is transmitted to a controller for processing. The controller senses the voltage change in the panel and computes the touch point.
On the other hand, a capacitive touch panel uses a capacitive sensor, for sensing the capacitive change generated by the static electricity combination between transparent electrodes and human touch so as to detect the coordinates through an induced current of the capacitive change. When a finger touches the touch panel, the current will continuously flow through the sensor so that the sensor can store electrons in both horizontal and vertical directions to form a precisely controlled capacitive field. When the finger touches a different location, the normal capacitive field of the sensor is changed by another capacitive field. At this point, the circuit disposed at each corner of the panel will compute the change of the electrical field and then transmit a signal of the touch event to the controller for processing. Compared to resistive touch panel, the capacitive touch panel shows better performance and more convenient to use. However, because the capacitive touch panel requires a more complicated manufacturing process and the circuit of driving IC is more complex, the cost and the technology development are not suitable for the medium and small size products.
FIG. 1 shows a schematic view of a conventional circuit of using a relaxation oscillator to achieve capacitive sensing. As shown in FIG. 1, a relaxation oscillator 101 charges and discharges a capacitor Cx periodically, wherein the oscillation frequency and the capacitance of capacitor Cx are related to the current for charging and discharging, i.e., CdV=Idt. In the case of constant current, the capacitance change of capacitor Cx will change the frequency of the relaxation oscillator 101. A frequency comparator 103 compares the output frequency Fro of the relaxation oscillator 101 and the reference frequency Fref of a fixed reference clock 102. Because the relaxation oscillator 101 charges and discharges the capacitor Cx and the impedance of Cx is usually high, the Cx in this approach is prone to interference by the ambient noise.
FIG. 2 shows a schematic view of a conventional circuit of using charge transfer to achieve capacitive sensing. As shown in FIG. 2, a charge transfer circuit includes a capacitor Csum, a capacitor Cx, a comparator 201, and three switches S1, S2, S3, connected in the following manner: one end of the capacitor Cx connected through switch S1 to a voltage source VDD and the other end grounded; one end of the capacitor Cx grounded and the other end connected through switch S2 to the end of the capacitor Cx connected to switch S1, the voltage of the capacitor Csum is Vsum; two ends of capacitor Csum connected through switch S3 to each other; the voltage Vsum is outputted to the positive input of the comparator 201 and the inverse input is a reference voltage Vref; and the output of the comparator 201 is Vo.
The operation of the above circuit is described as follows: discharging the capacitor Csum through switch S3 to the ground level; applying a non-overlapping clock to switches S1, S2 to gradually transfer the charge on the capacitor Cx to the capacitor Csum, therefore, the voltage Vsum increasing gradually; when voltage Vsum exceeding reference voltage Vref, the output Vo rising from low level to high level, as shown in FIG. 3; and computing the time Tcf starting from Csum discharging to Vo transition. The higher the capacitance of the capacitor Cx is, the shorter the transition time is. The advantage of the above circuit is to provide better resistance to interference by the ambient noise. The capacitor Cx maintains at a low impedance during charging. During charge transfer, the capacitor Csum has a larger capacitance, which is often at least hundreds of times of the capacitance of the capacitor Cx. Thus, the capacitor Csum is also a low impedance component. Therefore, a better resistance to the RF interference can be obtained when the capacitor Cx maintains at a low impedance. On the other hand, the disadvantage of the above circuit is that the capacitance of the capacitor Csum must be hundreds or thousands of time of the capacitance of the capacitor Cx (for example, a few hundred of pF to nF), therefore, it is difficult to make the circuit as an integrated circuit, i.e., un-integratable. Another disadvantage is the way the charge transfer is conducted. The accumulation of the voltage Vsum is not linear, instead, the relation between the voltage Vsum and the capacitance of the capacitor Cx is an exponential relation. In other words,
      v    sum    =            VDD      ⁡              (                  1          -                      ⅇ                                          -                N                            ⁢                              Cx                Csum                                                    )              .  It is also less linear to compute the difference of the capacitance of capacitor Cx.
Therefore, it is imperative to address the disadvantage of the above technology.